posting powerpoint slides in asp.net page

The only way how you could do this is to convert your slides into some other format that can be displayed in browsers. For example, one solution would be to convert your powerpoint to pdf file in c#, or to an image file or to something else.
Here is a small sample code that uses a powerpoint library for .net framework:

var presentation = PresentationDocument.Load("Sample.pptx",
new PptxLoadOptions());
presentation.Save("Sample.pdf",
new PdfSaveOptions());

If you want to save to an image, then change the output file's name and use "ImageSaveOptions" instead or "PdfSaveOptions".
After you generate/convert your powerpoint of a compatible file format for browsers, then you just need to set the iframe's src to that file's location.